Method and device for producing fiber-reinforced components using an injection method

ABSTRACT

A method for producing fibre-reinforced plastic components made of dry fibre composite preforms by means of an injection method for injecting matrix material. Arrangement of the fibre composite preform ( 1 ) on one surface ( 11 ) of the preform ( 1 ) resulting in a flow promoting device ( 15 ), on a tool ( 3 ), creates a first space ( 10 ) by means of a gas-permeable and matrix-material-impermeable membrane ( 7 ) surrounding the preforms ( 1 ). Formation of a second space ( 27 ) situated between the first space and the surroundings by means of a foil ( 19 ) which is impermeable to gaseous material and matrix material, is provided, with removal by suction, of air from the second space ( 27 ) resulting in matrix material being sucked from a reservoir into the evacuated first space ( 10 ) and with the flow promoting device ( 15 ) causing distribution of the matrix material above the surface ( 11 ) of the preform ( 1 ) facing said flow promoting device ( 15 ), thus causing the matrix material to penetrate the preform ( 1 ) vertically.

[0001] The invention relates to a method for producing fibre-reinforcedplastic components made of dry fibre composite preforms by means of aninjection method and subsequent low-pressure curing, as well as a devicefor implementing this method.

[0002] Such methods use dry fibre composite preforms in order to producecomponents with geometric shapes that may be unwindable, non-unwindableor not completely unwindable. The dry fibre composite preform can be awoven fabric, a multi-axis interlaid scrim or a warp-thread reinforcedunidirectional preform. The above-mentioned preforms are used in theproduction of components made of fibre-reinforced material; theyrepresent an intermediate process step before infiltration by resin andcuring take place.

[0003] Such a method is known as a so-called resin film infusion (RFI)method wherein dry carbon fibres, carbon fibre woven fabrics or carbonfibre interlaid scrim are placed in a curing device before a specifiednon-liquid quantity of resin film is applied to them from the outside.The curing tools equipped and evacuated in this way are subsequentlycured in an autoclave or another pressure receptacle by exposure totemperature and pressure. The use of pressure receptacles and theassociated complex tools that are necessary are however very expensive,rendering such methods complex also in regard to temperatures andpressures to be maintained. The scope of application of such methods isthus limited.

[0004] Furthermore, the use of dry preform components is known fromDE-OL 198 13 105 A1 which discloses a method for producing fibrecomposite components wherein the fibres and the matrix material areformed in a tool, forming a mould cavity, said tool comprising at leasttwo parts, with the air situated in the mould cavity being able toescape. In this arrangement, a porous membrane is placed into the mouldcavity, in front of the apertures, with the pores of said porousmembranes being of such a size that air can be evacuated withouthindrance while the matrix material is retained in the mould cavity.

[0005] The solution proposed in DE-OL 198 13 105 A1 does not involve anyapplication of pressure. However, it is associated with a disadvantagein that the size of components that can be produced with this method islimited, because the matrix material can be introduced into the fibres,i.e. into the preforms, only in a limited way, provided a central matrixfeed bush has been provided, because the matrix has to flow along thepreform plane, i.e. along the fibres. Due to the distance to be coveredand the resistance put up by the material, this direction of flowcreates the largest flow resistance to the matrix. Thus, impregnationalong the length of material flow is limited. As an alternative, DE-OL198 13 105 A1 provides for the matrix to be put in place over an area.To this effect, resin reservoirs, situated on the component surface, areused, which require their own expensive resin supply device up to thepreform, thus at every position posing the risk of a leakage (risk ofrejects).

[0006] There is a further disadvantage in that this method can meet veryexacting quality standards of the component to be produced only to alimited extent. This is because as a result of the potential resinpassages through the vacuum foil and the membrane up to the preformsurface, matrix material can penetrate through the membrane in manylocations of the component, thus sealing off said membrane from above.In this case, air evacuation no longer functions and pores form withinthe laminate, because of the reaction during the curing process (e.g. asa result of trapped air, chemical separation, volatile components etc.).Such pores, which can negatively affect the quality of the component,cannot be eliminated.

[0007] Other known low-pressure methods such as for example VARI(application unknown, DLR) do without a membrane and two-part vacuumchambers. They avoid pore formation by process management of the vacuumand temperature outside the boiling range of the matrix material. Inthis way no pores arise in the component. However, there is adisadvantage in that temperature and vacuum management must be veryexactly adhered to at every position of the component, to avoid locallyentering the boiling range of the matrix, with subsequent local poreformation. In the case of large components, such precise processmanagement can only be realised with considerable difficulty andexpense. This method has a further disadvantage in that as a result ofpermanent suction to maintain a vacuum, matrix material can be drawnfrom the component, which again can create pores. Furthermore, a resintrap or similar is necessary so as to prevent damage to the vacuum pumpas a result of any matrix material issuing.

[0008] It is thus the object of the invention to create a method forproducing fibre-reinforced plastic components made of dry fibrecomposite preform by means of an injection method, as well as a devicefor implementing the method, said method being suitable even for largercomponents, and allowing process management which is as simple aspossible while at the same time making it possible to achieve goodcomponent quality.

[0009] This object is met with the characteristics of the independentclaims. Further embodiments are disclosed in the subordinate claims.

[0010] With the solution according to the invention, it is possible inparticular to achieve top quality components. This is in particularadvantageous in the case of highly stressed structural carbon fibrereinforced plastic components in the aircraft industry. Typicalparameters indicating the quality of the components include e.g. thenumber of pores-within the cured carbon fibre reinforced plasticlaminate and the temperature resistance expressed in theglass-transition temperature of the matrix material after the process.

[0011] The solution according to the invention applies in particular tothe production of composite reinforced plastic components containingcarbon fibres, glass fibres, aramide fibres, boron fibres or hybridmaterials whose geometric shapes may be unwindable, non-unwindable ornot completely unwindable. The solution is also suitable for theproduction of non-stiffened or stiffened, large-area planking fields,plastics tools or tapered overlap repairs of damaged fibre compositecomponents. Stiffening can be achieved by so-called integral stiffening(profiles made of carbon fibre reinforced plastic etc., profilescomprising a combination of sandwich and carbon fibre reinforced plasticetc.) or stiffening can be achieved by a typical sheet-like sandwichstructure.

[0012] The solution according to the invention provides a cost-effectivemethod for producing fibre reinforced components, plastics tools andrepair patches for tapered overlap repairs using vacuum injectiontechnology and curing in a vacuum, without the use of an autoclave orwithout the use of overpressure.

[0013] Below, the invention is described with reference to the enclosedFigures, as follows:

[0014]FIG. 1 a diagrammatic view of a section through the deviceaccording to the invention, said device being suitable to implement themethod according to the invention;

[0015]FIG. 2 a typical design of an integrally stiffened component as asandwich hat-profile variant in the device according to FIG. 1;

[0016]FIG. 3 a typical design of an integrally stiffened component as aT-profile variant in the device according to FIG. 1;

[0017]FIG. 4 a typical temperature and vacuum gradient over time, for aso-called 350° F. system;

[0018]FIG. 5 a typical temperature and vacuum gradient over time, for aso-called room temperature (RT) system; and

[0019]FIG. 6 a diagrammatic view of a section through an alternativeembodiment of the device according to the invention.

[0020] The device shown in FIG. 1 shows the component or dry fibrecomposite preform 1 to be produced, which is arranged on a tool 3, forexample by means of a mounting 5. The component or laminate can be areinforced plastic component containing carbon fibres, glass fibres,aramide fibres, boron fibres or hybrid materials whose geometric shapemay be unwindable, non-unwindable or not completely unwindable. Thecomponent or laminate is in particular suitable for the production ofnon-stiffened or stiffened, large-area planking fields, plastics toolsor tapered overlap repairs of damaged fibre composite components.Stiffening can be achieved by so-called integral stiffening (profilesmade of carbon fibre reinforced plastics etc., profiles comprising acombination of sandwich and carbon fibre reinforced plastics etc.) orstiffening can be achieved by a typical sheet-like sandwich structure.The shape of tool 2 is suitable for accommodating the component 1 or ifnecessary the mounting 5. Said tool 2 can be made from various suitablematerials, e.g. wood, steel, sheet metal, glass or the like.

[0021] Component 1 is covered by a semi-permeable membrane 7 which isgas-permeable but which prevents penetration of matrix material. Outsidethe circumferential area 8, the membrane 7 is sealed as closely aspossible to the component 1 by means of a seal 9 which seals the firstspace 10 formed by the membrane 7 and the mounting 5 or the tool surface3. As an alternative, the membrane 7 can also surround the entirecomponent as shown in FIG. 6. This can be achieved by means of the seal9 (FIG. 6) or without such a seal, by designing the membrane 7 in asingle piece. Between the component 1 and the membrane 7, above theentire surface 11 of the component 1 facing the membrane 7, a peel ply13 (optional) and a spacer as a flow promoting device 15 can bearranged. The peel ply 13 and the spacer serve to hold the membrane 7 ata distance from the surface 11 of the component 1. The flow promotingdevice 15 can be a type of grate or screen or a stiff woven or knittedor braided fabric which does not significantly compress when a vacuum isapplied. Said fabric comprises for example metal, plastics orsemi-finished textile materials.

[0022] The arrangement 17 comprising mounting 5, component 1, membrane 7with seal 9 as well as peel ply 13 and flow promoting device 15, iscovered by a foil 19 which is impermeable to gas. Around thecircumference of the membrane 7, said foil 19 is sealed on the tool 3 bymeans of a seal 21 so that the second space or interior space 27 whichis formed by the surface 23 of the tool 3 and the internal wall 25 ofthe foil 19, is sealed off from the surroundings. A ventilation fabric32, for example a woven glass fabric or a fibrous web or similar, isplaced between the foil 19 and the membrane 7. This ventilation fabric32 leads the air and gasses, which were removed by suction through themembrane, from the interior space 25, along the membrane surface, forremoval by suction through the vacuum pump 29. This interior space 27can be evacuated by means of a vacuum pump 29 (not shown) and arespective gas pipe 31 which leads into the interior space 27. Inaddition, a second pipe 33 leads into the interior space 27, throughwhich pipe 33 matrix material and in particular resin, can be introducedinto the interior space 27.

[0023] To feed matrix material into the component 1, hoses or pipes 33which are connected to a resin reservoir (not shown) lead into a space25 situated in the first space 10. The tool and the reservoir for thematrix material are located on hot plates, within a heated chamber,within a heatable liquid (oil bath or similar) or within a controllableoven, if the selected resin system requires thermal treatment duringinjection.

[0024] The foil 19, the peel ply 13, the membrane 7, the ventilationfabric 32 and the flow promoting device 15 all must be resistant, forthe duration of the process, to the matrix systems used. In additionthey must also be resistant to the temperatures which occur during theprocess. Depending on the particular geometric shape to be produced,placement onto such a shape by stretching, fold formation or similarmust be possible.

[0025] The foil 19 is a gas-impermeable state-of-the-art vacuum membranewith the characteristics mentioned above. Its task is to seal off thesecond space 27 from the surroundings. Typical materials for this arefoils or rubber membranes. Examples for a 180° C. (350° F.) applicationinclude for example foils based on PTFE, FEP etc. Other materials may beconsidered, depending on the selected matrix system and its specificcuring temperature, taking into account the above-mentionedrequirements.

[0026] The peel ply 13 serves to facilitate separation (by peeling),after completion of the process, of the flow promoting device 15 filledwith matrix material from the component 1, because all the processmaterials mentioned are only used as auxiliaries in the production ofthe component 1. The peel ply 13 is designed to resist permanentconnection with the matrix material and the surface of the component.This is achieved by a particular surface structure of the peel plyand/or by additional non-stick coatings (such as for example PTFE,silicon or similar). Typical materials are for example woven glassfabrics, woven nylon fabrics or similar. The peel ply must begas-permeable and also permeable to matrix material in both direction.

[0027] The membrane 7 is a semi-permeable membrane e.g. made of atechnical plastic material which meets the process conditions as far astemperature resistance and media resistance are concerned. Furthermore,this membrane is gas-permeable but impermeable to liquids withviscosities that are comparable to water. This behaviour is achieved bygas-permeable pores situated in the membrane, said pores beingdistributed on the surface of the membrane over a greater or a lesserarea. The size of the pores is selected such that the matrix systemcannot penetrate them. The thickness of the membrane is in the range oftenths of a millimetre. Adequate flexibility for draping and forming isprovided by the use of typical plastic materials.

[0028] The ventilation fabric 32 above the flow promoting device 15serves to convey the air and other volatile components sucked throughthe membrane, for removal by suction to the vacuum pump 29. Thismaterial can comprise any material as long as it provides adequatetemperature resistance and media resistance to the materials necessaryduring the process, and as long as the conveyance of air in longitudinaldirection is possible. Fluffy mats, woven fabrics, knitted fabrics,braided fabrics and similar are used for this purpose, whereby saidarticles can be made from metal, plastic or other materials.

[0029] The flow promoting device 15 enables distribution on the surfaceof the component 1, of the matrix material which reached space 25 viathe matrix supply pipe. The flow promoting device 15 thus assumes thefunction of a flow channel. The flow promoting device 15 must maintain aminimum thickness when subjected to the vacuum build-up of foil 19, soas to enable such material flow. It is thus a spacer which forms a flowchannel between the membrane 7 and the component 1. The flow promotingdevice can be a braided fabric, a woven fabric, a knitted fabric orsimilar, with, if at all possible, a wide-meshed structure so as tocreate little flow resistance. Any materials can be used, e.g. metal orplastic or similar, as long as the above-mentioned common minimumrequirements (temperature and media resistance) are met. To support thetransport of the matrix, the matrix supply pipe 33 can reach as far asrequired into the first space 10. One branch or several supply pipes arepermissible. Within the first space 10, this matrix supply pipe maycomprise apertures, for example holes, transverse slots, longitudinalslots or similar. These assist resin transport in the flow promotingdevice.

[0030]FIGS. 2 and 3 show the device according to the invention as shownin FIG. 1, except that each figure shows a different component 1. Thereference numbers for components of the same function are the same inthese figures. It is evident that the device according to the inventionis suitable for components of almost any shape. FIG. 2 diagrammaticallyshows a planking field (component 1) which in one direction is stiffenedby means of hat profiles. These hat profiles comprise a foam core 35 ora core formed from any material, with a closed surface and with dryfibre composite preforms 34 placed thereon, said dry fibre compositepreforms being hat-shaped. The fibre composite preforms 34 are made frommaterials which are identical or similar to those of component 1. Thefoam core 35 and the preforms 34 form part of component 1.

[0031] The component 1 of FIG. 3 is also a planking field which inlongitudinal direction is stiffened by one or several T-profiles 36.Component 1 which is to be produced according to FIG. 3 thus comprisesthe individual components 1 and 34. The T-profiles 34 are made frommaterials which are identical or similar to those of component 1. Inaddition, this component variant requires a support 37 for fixing thedry T-profiles 36 which in their non-impregnated state are unstable.These supports 37 can be made from typically rigid or semi-flexible toolmaterials such as e.g. metal, wood, rubber, plastic etc. Since there isdirect contact with the matrix material, this material of the supports37 must keep its form in relation to the matrix material during theprocess.

[0032]FIGS. 4 and 5 show typical gradients of various resin systemclasses as a vacuum gradient 91 and a temperature gradient 92, with thegradient shown in FIG. 4 relating to a 350° F. system and the gradientshown in FIG. 5 relating to an RT-system.

[0033] The temperature and vacuum gradients can be broken down into atleast two phases, the injection phase 101 and the curing phase 103. Atempering phase 102 may be provided after these phases. In the injectionphase 101 the temperature is lower than in the curing phase 103.

[0034] The temperature gradient and the vacuum control are such that thecured component is of optimum quality with few to no pores and asuitable fibre volume fraction being achieved. The specifications fortemperature are determined by the materials requirements of the matrixmaterial. Irrespective of the matrix material selected, during theentire process right through to curing, i.e. the condition in which thematrix material has changed its aggregate state from liquid toirreversibly solid, the vacuum can be kept at a constant level. Normalvalues and tolerances which must be observed include for example 1 to 10mbar (absolute pressure, near the ideal vacuum). After curing 103, it isno longer necessary to maintain a vacuum. The necessary temperaturegradients are characterised as follows: during the injection phase 101at full vacuum, a temperature is required which is determined by theviscosity curve of the matrix material. The temperature is selected suchthat the matrix material becomes liquid enough to reach the interiorspace 25 via the supply pipe 33 by means of vacuum suction. This is theminimum temperature necessary for the process. At the same time thistemperature must not be so high as to cause curing (loss of viscosity,solid state of the matrix). Therefore (depending on the matrix materialselected), the process temperature is set such that injection ispossible (slight viscosity) and that the remaining time to curing forthe injection, i.e. near-complete filling of the interior space 25 withmatrix material is adequate (technical term e.g. gel time). Typically,the necessary viscosities during the injection phase range e.g. from 1to 1000 mPas, Typical temperatures for a 350° F. (180° C.) system aree.g. 70 to 120° C. for the injection phase 101, approx. 100 to 180° C.for the curing phase 103, and values of approx. 160 to 210° C. for thetempering phase 102.

[0035] For selected matrix materials, e.g. RT matrix materials, thefollowing variant is advantageous: injection temperature 101 equalscuring temperature 103 equals tempering temperature 102.

[0036] The vacuum is established before the injection phase 101 (FIG. 4)or before it. In the method according to the invention a vacuum whichtypically ranges from 1 to 10 mbar, is generated for injection, saidvacuum extending to completion of the curing phase. Said vacuum shouldnot be reduced.

[0037] The method according to the invention is described below:

[0038] Dry materials (e.g. carbon fibre reinforced interlaid scrim,woven fabric, etc.) are positioned as specified in the design, and thusa laminate structure is formed from the individual layers of preform.The tool has been pre-treated to separate, i.e. by means of releaseagents or separating foil and peel ply (altogether this constitutes thedesign 5 on the underside of component 1). This prevents sticking of thematrix material to the tool and allows removal of the component(stripping) from the tool surface. The dry material of the component 1preferably comprises the peel ply 13. In addition, a so-called flowpromoting device 15 is simply placed above this construction. In thecase of complex components, local lateral attachment, e.g. withtemperature-resistant adhesive tape, is advantageous. The membrane 7,which is air-permeable but not liquid-permeable, is placed onto thisflow promoting device 15 and sealed off by means of the seal 21. Thenthe ventilation fabric 32 is placed on the membrane 7 and sealed offfrom the surroundings by means of the foil 19 and the seal 21. Duringthis procedure, the matrix supply pipe 33 and the vacuum pipe 29 are putin place with commercially available bushings and seals as shown in FIG.1.

[0039] After placement of the above-mentioned materials and the foil orvacuum film 19, the first space 10 is evacuated using the vacuum pump.At the same time a reservoir containing matrix material is connected tothe system, to introduce matrix material into the first space 10. Thevacuum results in a drop in pressure so that the matrix material issucked from the reservoir into the evacuated first space 25. After this,the matrix material flows through the flow promoting device 15 and thesupply pipe 33 and is distributed on the surface of the component, moreor less unhindered, and almost irrespective of its viscositycharacteristics. Any air present is disposed of through the membrane 7,as a result of permanent evacuation, by suction, of the interior space27. There is no flow of matrix material within the laminate constructionwhich is characterised by considerable flow resistance. Instead, theinfiltration of matrix material takes place from the component surfacevertically downward into the laminate. The maximum flow path at eachposition of the component is thus directly related to the componentthickness at this point. The flow resistance is thus minimal.Consequently it is now possible to use resin systems which due to theirviscosity were hitherto unsuitable for infiltration, and it is possibleto create components of large dimensions.

[0040] Membrane 7 serves the purpose of preventing the occurrence oflocal air cushions. If for example the flow fronts which form, close up,creating a closed air cushion in component 1 of the interior space 25without binding to the vacuum outflow of the air, no resin can flow intothis air cushion. A defect (no impregnation) would be the result. Theair-permeable membrane 7 prevents this effect because at every positionin the component, air can always move vertically to the surface, throughthe membrane, into a resin free space which can be ventilated, of thevacuum build-up 27. From there, above the membrane 7, the air is removedby suction, via the vacuum connection 29 by means of the ventilationfabric 32. The membrane is resin-impermeable. There is thus no need formonitoring the flow fronts because the process of impregnation isself-regulating. The degree of impregnation is directly related to thequantity of resin supplied and thus available to the process, as well asbeing directly related to the quantity of fibre supplied.

[0041] As soon as complete impregnation has taken place, curing iscarried out at a suitable temperature while the vacuum is maintained atthe same level. In known processes, the bubbles arising as a result ofthe chemical process (matrix boiling, volatile components etc.) wouldlead to pore formation in the finished component. This is now preventedby the membrane 7, because permanent ventilation vertical to the surfaceof the component takes place through the membrane.

[0042] On completion of curing, the component can be stripped. Thismeans that all process materials are removed from component 1, e.g. bypeeling them off manually, and the component can be separated from thetool 3. Depending on requirements, the now stripped hard component withpreforms impregnated with matrix, can be subjected to a pure thermalafter-treatment (tempering in step 102). Tempering can also take placeprior to stripping, but this is not necessary. Tempering after strippingwill reduce the time during which the tool is tied up.

[0043] The maximum size of components which can be produced with themethod according to the invention is almost unlimited. A natural upperlimit is more likely to be dictated by considerations associated withhandling of the component (transport etc.) rather than with the methoditself. There is no minimum size for these components. The maximumachievable thickness depends on the resin types used and the availableinjection time. This injection time is determined by economic ratherthan technical limits. Other undesirable side effects such as forexample an exothermal reaction during curing, depend only on the resinsystem rather than on the method.

[0044] In summary, the invention relates to a method for producingfibre-reinforced plastic components made of dry fibre composite preformsby means of an injection method for injecting matrix material. In thismethod, removal by suction, of air from the second space 27 takes place,resulting in a pressure drop from the first space 10 to the second space27, with matrix material being sucked from the reservoir into theevacuated first space 10. Because of the flow promoting device 15, saidmatrix material enters the preform 1 vertically, in a distributedmanner, above the surface 11 of the preform 1 facing the membrane 7. Bycombining the functions of distributing the matrix material above thecomponent surface through the flow promoting device, and the possibilityof area-like ventilation above the component, as well as the flowpromoting device, through the membrane foil, the desired quality isachieved with curing in a vacuum, without the use of overpressure.

1. A method for producing fibre-reinforced plastic components made ofdry fibre composite preforms by means of an injection method forinjecting matrix material, involving the following steps: 1.1 Arrangingthe fibre composite preform (1) on a tool (3), with a flow promotingdevice (15) being arranged on one surface (11) of the preform (1). 1.2Creating a first space (10) by means of a gas-permeable andmatrix-material-impermeable membrane (7) arranged at least on one sidearound the preform (1), whereby matrix material can be fed into thefirst space (10). 1.3 Creating a second space (27) adjacent to the firstspace (25), with the second space (27) being delimited from thesurroundings by means of a foil (19) which is impermeable to gaseousmaterial and matrix material, with said foil (19) being sealed off fromthe tool (3). 1.4 Removal by suction, of air from the second space (27),with matrix material being sucked from the reservoir into the evacuatedfirst space (10) and with the flow promoting device (15) causingdistribution of the matrix material above the surface (11) of thepreform (1) facing said flow promoting device (15), thus causing thematrix material to penetrate the preform (1) vertically.
 2. A device forproducing fibre-reinforced plastic components made of dry fibrecomposite preforms by means of an injection method for injecting matrixmaterial, comprising a tool (3) for arranging the fibre compositepreform (1), a gas-permeable and matrix-material-impermeable membrane(7) arranged at least on one side around the preform (1) creating afirst space (10) into which matrix material can be fed; a flow promotingdevice (15) arranged on a surface (11) of the preform (1); a secondspace (27), sealed off from tool (3), adjacent to the first space (25),which second space (27) is delimited from the surroundings by means of afoil (19) which is impermeable to gaseous material and matrix material;with removal by suction, of air from the second space (27) resulting inmatrix material being sucked from the reservoir into the evacuated firstspace (10) and with the flow promoting device (15) causing distributionof the matrix material above the surface (11) of the preform (1) facingsaid flow promoting device (15), thus causing the matrix material topenetrate the preform (1) vertically.